Compared with the two-space extension, the three-space extension has two advantages, which can be explained using go concepts, and one clear disadvantage.
To start with the disadvantage. White can invade at here, and (unlike the case of the two-space extension) Black cannot force a connection by immediate means. What Black usually does is to jump out, for example at a, enlarging the scale of the struggle; or to play at b (possibly leading to a later sacrifice), to make the most of tactics locally. Therefore this length of extension isn't easily handled in all positions.
One major reason to choose to play the three-space extension can be seen in this sequence. If White is about to play the diagonal attachment at , Black's marked stone ends up on an ideal point after the proper answer
(see extension from a wall). One line closer is over-concentrated, that is, worse, from the point of view of good development.
The second possible advantage is if Black is seeking to play lightly - stay out of trouble.[1] This type of sequence is appropriate if White is strong in the upper right, as here. In that case Black a, White b might be good for White, and Black is better allowing in return for
.
Here Black can be quite happy with his development.
In some contexts completing the shape with Black at a (instead of jumping out at ) might be better.
There are pages dealing with invasions in these extensions on the three-space extension invasions path.
[1]
, approaching White's wall, is problematic, while extending only to a instead of
allows White to play at b.
-
press against
, preparing for a safe extension to
. This sequence avoids the problems of an immediate extension.
Charles This idea of Bill's, to use the 3-5 point low approach, press, is a bit more subtle than may seem at first glance. For example, is held back one line, giving White much less chance of fighting on this side.